1. Field of the Invention
The present invention relates to a variable magnification optical system usable in a video camera, an electronic still camera, and the like, and to an imaging apparatus. In particular, the present invention relates to a variable magnification optical system appropriate for a surveillance camera, and which is usable in a wide wavelength band from a visible range through a near-infrared range, and to an imaging apparatus including the variable magnification optical system.
2. Description of the Related Art
Conventionally, surveillance cameras were used to prevent crime, to record and the like. An optical system for such a surveillance camera needs to be structurable in small size and at low cost. Further, the optical system needs to have a large relative aperture to identify a subject even in low illumination photography conditions, and to have a high variable magnification ratio and high optical performance.
Optical systems for surveillance cameras have been proposed, for example, in U.S. Pat. No. 7,450,315 (Patent Document 1) and Japanese Unexamined Patent Publication No. 2009-237400 (Patent Document 2). In both of the optical systems disclosed in Patent Documents 1 and 2, the optical system consists of a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, and a fourth lens group having positive refractive power in this order from an object side. The first lens group and the third lens group are fixed and the second lens group and the fourth lens group move during magnification change. The first lens group consists of three lenses, which are a positive lens, a positive lens and a negative lens arranged in this order from the object side.